Depositing a single monomolecular layer on material surfaces has been the subject of numerous investigations. The importance of such technology spans multiple disciplines, including heterogeneous catalysis, material separation and filtration, functionalization of polymers with monomolecular coatings, and electronics. One approach to forming monomolecular layers includes formation of chemisorbed organic monolayers, which relies on the chemical reaction between the activated surface and the reactive organic molecule. To form a stable monomolecular layer, the reaction must be complete, functionalizing all reactive surface sites, and form thermodynamically stable and non-hydrolytic surface bonds. Activated but only partially reacted surfaces readily interact with adventurous molecules, for example via van der Waals forces or chemical reactions. This causes monolayer contamination and/or degradation of hydrolytic surface bonds, even if they are thermodynamically stable.
One approach to achieve both complete surface coverage and stable attachment of a monolayer includes self-assembled monolayers (SAMs). SAMs utilizes molecules with symmetrical structures that can form ordered two-dimensional organic phases stabilized by the extensive van der Waals interactions. SAMs have been successfully applied on many flat inorganic surfaces, but was never successfully extended to rough interfaces. Typically, interfaces with roughness' comparable to or greater than the monolayer thickness (e.g., ˜2-10 nm) have disrupted two-dimensional van der Waals interactions and disordered organic phases. SAMs have also never been successfully extended to soft materials. The majority of soft materials (polymers and functional thin films) are too inert and their activation requires the use of chemicals, solvents or charged plasma species that degrade, dissolve or etch the material. The SAM approach also imposes several requirements on the symmetry and size of the molecules used to form the monolayer. For example, monolayers bearing large terminal functional groups are generally unstable due to the relatively low order in the organic phase resulting from steric constraints induced by the terminal functionality. In contrast, simple aliphatic thiols, silanes, and phosphonic acids form highly ordered and stable SAMs on metals and oxides that act as impervious barriers to organic and aqueous solutions. However such ordered monolayers are too inert and not amenable to traditional functionalization protocols to carry out a needed function. Self-assembly also precludes formation of uniformly mixed multicomponent monolayers. Literature examples show that even a ˜5 kcal/mol difference in the van der Waals stabilization energy of two different molecules leads to their phase segregation into distinctive mono-component domains when they are co-deposited together from the same solution. For example, co-deposition of homologous molecules (e.g., unbranched aliphatic thiols that differ in length only by 3 methylene units) results in a monolayer surface composition that does not correlate with the solution concentrations due to the preferential adsorption of molecules with stronger van der Waals interactions. This essentially precludes formation of any uniform multicomponent monolayers that have slightly different components.
Functionalization of chemically inert surfaces by attaching organic molecules via stable non-polar bonds have been reported. For example, stable H-terminated diamond, carbon nanotube and graphene sheets can react with organic alkenes and alkynes to form stable monolayers that are connected to the surface through C—CH2 and C—CH bonds. However, the formation of such monolayers relies on harsh hydroalkylation reactions that typically require chemical catalysis, specific solvents or high temperatures—conditions incompatible with many inorganic substrates, let alone with organic polymers and thin films.
There is a need for methods of depositing stable functional monolayers on inert supports that do not change in ambient or mild reactive environments. There is also a need for depositing a uniform monomolecular film whose packing densities are determined only by the molecular sizes and not by the intermolecular van der Waals interactions. There is also a need for monolayers that are free of conformational or pinhole defects. The compositions and methods disclosed herein address these and other needs.